Three-dimensional object digitizing has many applications. Automobile and aircraft designers use such technology to convert prototypes into computer model data. The data may then be used to determine the accuracy of the prototype with respect to the design, to ensure quality control during production, and so on.
Three-dimensional digitizers fall into two categories: contact and non-contact systems. Most contact systems employ either manually operated probes or automatic coordinate measuring machines (CMMs). These contact systems collect data one point at a time. Clearly, contact systems are neither practical nor economical for large-scale modeling projects such as automobiles or aircraft.
Non-contact systems employ optical techniques to obtain data, for example, lasers, moire interferometry, and patterned light. Laser digitizers illuminate either a small spot or a thin line of light (which is more than 100 times faster than the small spot illumination) on the surface of an object. A technique known as triangulation is then used to determine the location of points in three-dimensional space. Systems based on moire interferometry or patterned light can quickly capture a set of data consisting of 50,000 to 100,000 points.
Not every system is suitable for every candidate for three-dimensional digitization. There are a large number of factors that can affect the quality of the digitization, including the color and surface finish of the object, the shape of the inside and outside corners and edges of the object, the existence of holes and concavities in the object, and whether the object is inanimate or alive (e.g., a human). All of these factors make it difficult to develop systems that meet specific market requirements.
The utilization of a digitizer requires the generation and collection of data and the subsequent processing of the data. The collected data (in the form of coordinate points) produce what is known as a data cloud or a data explosion because of the potentially millions of bits of data. The data cloud is generated arbitrarily without any sensitivity to the surface topology of the object being scanned and digitized because of inherent limitations in the data-generating device. The data cloud is collected into a computer file, typically a very large and cumbersome computer file.
At this point, the data cloud does not represent any practical value. Accordingly, the user needs to perform rigorous and time-consuming work in order to translate the data cloud into a meaningful file format which represents the surface and the features of the scanned object. In processing the data cloud, the user attempts to extract the surface features (such as edges, depressions, circles, etc.) from the data cloud, which extraction facilitates data manipulation, including scaling, mirror imaging, tool path generation, finite element analysis, metamorphic transitions, optical special effects, and so on. This manipulation and extraction of surface features from the data cloud is one of the technological bottlenecks in the industry, one on which much time and effort has been centered for improvement.
The majority of conventional digitizers generate and collect data through one form or another of a technique known as triangulation. Referring to FIG. 14, triangulation is a technique that relies on the Pythagorean Theorem. A right triangle is defined by a calibrated distance A between a laser and a sensor, a transmitted laser beam B, and a received beam C. Triangulation techniques suffer from a number of drawbacks. For example, a "shadow" in the data may be produced if the received beam C is obstructed. Additionally, in order to produce accurate measurements, an angle .theta. between the transmitted beam B and the received beam C must be at least 30 degrees; accordingly, the physical dimensions of the moving scanning head and of the digitizer are functions of the size of the object being digitized. For example, if the digitized object has a two-foot-deep surface depression in its topography, the size of the triangulating probe will have to be approximately 16 inches. Such a large probe increases the mass of the scanning device and, therefore, the risk of mechanical instabilities for which exists a constant need to calibrate the scanner.
Turning to range finders in general, one of the most common in use today is police radar range-finders. Modern police radar range-finders use semiconductor lasers (as opposed to radar previously) to project a beam of light from which a measurement is derived. The devices may use the frequency shift (i.e., the Doppler effect) on modulation of the laser beam. Alternatively, the devices may transmit short pulses of light and measure the changing time of return of the pulse from which velocity of the target is calculated. Neither approach is able to provided highly accurate distances.
Other conventional range finders include the Geodimeter.TM. which is an electro-optical device that measures distance on the basis of the velocity of light. The approach used in a Geodimeter is to send out fixed frequency modulated light beams that are retroreflected back to the instrument where the variable phase of the return signal is measured to calculate distance. This has been the standard conventional approach to measure distance: sending out a fixed frequency signal and measuring the phase of the return signal.
In view of the above-mentioned drawbacks of conventional apparatus, it is an object of the present invention to provide methods and apparatus for measuring distance and/or digitizing objects which mitigate and/or obviate these drawbacks.
It is another object of the present invention to provide methods and apparatus which eliminate the limitations of existing scanning and range-finding devices.
It is a further object of the present invention to provide apparatus for measuring distance that is relatively small and portable.
It is still another object of the present invention to provide methods and apparatus for digitizing objects which are able to digitize an object in a matter of seconds rather than hours or days with conventional devices.
It is yet another object of the present invention to provide a distance-measuring apparatus which is significantly less expensive and, therefore, more widely applicable than conventional systems.
It is still a further object of the present invention to provide methods and apparatus for distance measuring in which optical signals for generating and collecting data are transmitted to and received from a target coaxially.
It is another object of the present invention to provide methods and apparatus for digitizing objects which significantly reduce the number of data points required to describe features of an object accurately.
It is yet a further object of the present invention to provide methods and apparatus for distance measuring and/or digitizing objects which perform data manipulation (e.g., curve fitting) on-the-fly. Accordingly, rather than generating a data cloud, a mathematical representation of data is output, which representation is user definable.
It is another object of the present invention to provide methods and apparatus for digitizing objects which do not require a coherent (i.e., single frequency) source of light, thereby eliminating the need for a retroreflector used in interferometry techniques. Accordingly, the present invention is able to measure objects with a wide range of surface qualities, eliminates the need for troublesome target-instrument mechanical alignment, solves shadowing problems, and removes the limits for distance measurements without increasing the size of the measuring device.
It is a further object of the present invention to provide methods and apparatus for digitizing and close-range distance measuring which provide highly accurate and absolute measurements for real-time quality control applications. Accordingly, the present invention may be integrated into manufacturing processes as one tool in the tool library of a computerized numerically controlled (CNC) milling machine. Such integration allows precision manufacturers (e.g., aerospace companies) to certify parts without removing the parts from the bed of the machine, thereby significantly increasing the speed of the manufacturing process.
It is still another object of the present invention to provide methods and apparatus for distance measuring and/or object digitizing which may be implemented in a multiple scanning- head system in which the multiple scanning heads are stationary. Accordingly, digitizing installations implementing the principles of the present invention are able to digitize large objects, for example, automobiles or airplane wings, in a matter of seconds and objects of any shape or configuration. Significant savings over conventional scanning systems are realized in hardware, manpower, and time.